Offshore platforms are generally built in onshore fabrication yards where the entire platform is fabricated in one piece. Once the platform is ready for installation offshore, the platform is loaded onto a launch barge which transports the platform to a predetermined installation site. The platform is then launched into the ocean and installed.
When the oil and gas industry seeks to develop petroleum reserves in deep water, the platforms are necessarily much larger and longer. Where platform specifications exceed the capability of existing launch barges and fabrication yard capacities, the platforms must be fabricated in sections of a size that can be handled by the launch barges available for a particular development. These sections are then launched individually and joined together offshore.
Currently, both vertical and horizontal offshore joining methods are utilized. Joining the sections of an offshore structure using these existing methods is limited by a number of factors, including weather conditions (wind, wave, current); safety; costs associated with the mooring equipment (if horizontally joined) or lowering equipment (if vertically joined) and the launch barges and other vessels necessary to perform the joining of the sections; the number of vessels required; and the time required to launch, moor or lower, and join the sections. In addition, few presently available vessels have the capacity required to join sections of an offshore structure designed for deep water installation using existing joining methods. All of these limitations are alleviated by the present invention.
Industry has utilized a multiple point mooring concept to accomplish horizontal joining of offshore structures. Typically, two construction vessels and three mooring barges (or one construction vessel and four mooring barges) are required. Each section of the offshore structure is transported on an individual launch barge to the joining site where it is launched. The sections are then attached to barges which are moored to the ocean floor. Alternatively, the sections themselves may be moored to the ocean floor from their four corners independent of the construction vessels. By independently mooring the sections, the joining operation can withstand storms of much greater intensity than if the construction vessels remained attached to the sections. The independent mooring procedure is possible because the construction vessels are self-equipped to moor themselves to the ocean floor. Consequently, the need to purchase and design a mooring system to handle the additional loads that would be created by the construction vessels if they were attached to the moored sections is eliminated.
The major disadvantages of these horizontal joining procedures are the number of launch barges required, the expense of the mooring barges and/or the complex mooring array, and the limited ability of the construction vessels to withstand high intensity storms. These high intensity storms also restrict the procedures to a very short time frame in rougher ocean environments such as the Gulf of Mexico. Because of the restricted time frame, the demand for the launch vessels needed to perform a joining often exceeds the supply.
The vertical joining procedure addresses the problem of demand for launch barges and the associated costs as it only requires one launch barge. With this procedure, the launch barge is used to transport each section to the joining site. When the first section arrives at the joining site onboard the launch barge, a construction vessel is required to lower and permanently secure it to the seafloor after it is launched. The section may be secured using piles, or may be attached to a preinstalled foundation or base assembly. The construction vessel must then remain idle at the site while the launch barge returns to the fabrication yard to load and launch the other sections at the joining site. The cost savings derived from elimination of one launch barge is largely offset by the cost associated with the idle construction vessel.
Both horizontal and vertical joining procedures must be designed to withstand loads exerted by the offshore environment. Accordingly, a procedure is desired that will reduce hydrodynamic forces exerted on the mooring system. If the number of mooring lines and other equipment (such as mooring barges) can be reduced, the joining procedure will be simplified and hydrodynamic forces exerted on the mooring system lowered, thus leading to lower mooring loads and reduced equipment requirements. A simplified procedure would also reduce the costs associated with the joining procedure. A need also exists to design a system that can withstand strong weather conditions and be used virtually year round.
The present invention is designed to withstand weather conditions more severe than joining procedures envisioned to date can withstand, eliminate the need for more than one launch barge, reduce construction vessel time, reduce the amount of equipment necessary to perform the mooring and joining procedure, reduce the hydrodynamic loads exerted on the mooring system, and place no seasonal restrictions on when the joining procedure may be performed. Furthermore, the present invention addresses the need for a method to join sections of an offshore platform designed for deep water installation using currently available vessels.